The Egyptian official said that a brother and sister, both Egyptians with British nationality, and their Egyptian cousin were apprehended at a subway station Saturday night after a passenger told police they were speaking in English about burning homes and businesses on the anniversary of the January 2011 uprising which led to the ouster of Hosni Mubarak.
The three denied the accusations and the case was referred to prosecutors, said the official, speaking on condition of anonymity because he was not authorized to brief reporters.
The British Embassy in Cairo has been closed to the public since last week because of security fears.
Since the 2011 uprising, successive governments have stoked fear of foreigners, warning the public that a myriad of "foreign hands" have been behind attempts to destabilize the country.
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At one point in 2012, a public service announcement was played on national television warning Egyptians against talking to foreigners, including in public places, for fear they would be spies.
A woman in the cafe screamed after hearing their conversation in English and Arabic and reported them to nearby security forces.